1,721,000 research outputs found
End-to-end TCP Enhancements Performance on Satellite Links
No full textAlthough TCP has proved very effective and robust for
many years, its performance on emerging heterogeneous
networks is challenged by the impairments originated by the
presence of radio links. In particular, satellite communications
are affected by long RTTs and possibly also by random segment
losses, which can severely affect end-to-end performance. To cope
with these problems, several TCP enhancements have been
presented in the literature. The paper aim is to investigate the
effectiveness of such modifications, when applied to TCP satellite
connections. In particular, the analysis focuses on some emerging
proposals, namely TCP Hybla, developed by the authors, and
TCP Westwood, examined here in three variants. They are
compared with three well established TCP variants, such as
NewReno, SACK and Vegas, taking into account both a pure
satellite environment and more challenging, but also perhaps
closer to reality, heterogeneous network. Performance is assessed
by means of ns-2 simulations considering goodput, fairness and
friendliness as performance metrics. Results show that large
performance improvements may be achieved by some of the
considered TCP enhancements, without infringing the end-to-end
semantics of this protocol
Satellite communications: From PEPs to DTN
Full text linkDelay-/Disruption-Tolerant Networking represents an innovative way to cope with satellite communications impairments. In this view, the paper presents an in-depth analysis of implications of a DTN approach to satellite communications, focusing on these fundamental aspects: network architecture, security, and Quality of Service (QoS). For each topic, commonalities and differences between DTN and Performance Enhancing Proxies (PEPs) are highlighted, to show that the DTN architecture can be seen as a promising evolution of PEPs, at present the most widely adopted architecture. The analysis shows that DTN can effectively improve PEPs functionalities in the presence of intermittent and disruptive channels and/or a high level of network heterogeneity. In particular, DTN offers the possibility to operate with intermittent channels, a better resilience to channel disruptions, the possibility to implement both end-to-end and hop-by-hop security, and a greater flexibility in the use of advanced QoS techniques
DTNperf_2: a Performance Evaluation tool for Delay/Disruption Tolerant Networking
No full textDelay/Disruption Tolerant Networking (DTN) has recently been proposed as a very effective solution to cope with the problems posed by challenged networks (long delays, disruptions, absence of end-to-end connectivity, etc.). In order to assess DTN performance in such scenarios, it is essential to be able to rely on specific evaluation tools. In this paper we present DTNperf_2, which is a client-server evaluation tool designed to assess goodput and to provide logs in DTN bundle layer architectures. The aim is to facilitate the adoption of this tool by the DTN research community. DTNperf_2 features, design and usage are discussed in detail. Moreover, an example of use is given, to demonstrate its capabilities. DTNperf_2 is included in the official DTN suite released by DTNRG
DTN meets smartphones: future prospects and tests
No full textAlthough first conceived for the Interplanetary Internet, DTN (Delay-/Disruption-Tolerant Networking) architecture has successively enlarged its application scope to embrace all “challenged networks”, i.e. networks where the TCP design key assumptions of short RTT, no disruptions, and continuous end-to-end path availability are challenged. The distinctive features of DTN architecture seem ideal for the connectivity problems of modern smartphones. These are both PCs and cell phones and are characterized by a wide range of different communications technologies, including Bluetooth, WiFi, and advanced 3G radio interfaces, like UMTS HSPA (High-Speed Packet Access). The aim of this paper is to show that it is both feasible and useful to adopt DTN architecture on these advanced devices, in addition to the usual TCP/IP protocol stack. To this end, a series of tests is presented and discussed in the paper, considering various communication interfaces, 3G in particular. Tests were greatly facilitated by the introduction onto the market of some smartphones running Linux based operating systems, like Maemo, which allowed the authors the direct use of “official” DTN code (i.e. the DTN2 reference implementation and the DTNperf_2 evaluation tool)
TCP, PEP and DTN Performance on Disruptive Satellite Channels
No full textFixed GEO satellite communications are impaired by long RTTs and the possible presence of packet losses on the satellite radio channel. Moreover, when the satellite receiver is mobile, short and long disruptions due to line of sight obstructions can cause further performance deterioration. In this paper, we present a preliminary assessment of disruption impact on performance comparing different approaches, such as end-to-end TCP, PEP based on TCP splitting and DTN. Performance is evaluated on a Linux testbed, by emulating the satellite link interruptions caused by tunnels on a real railway line. Results show that good performance can be achieved by all of the approaches considered provided that on the satellite channel an efficient TCP variant, like Hybla, is adopted; however, DTN offers best performance. DTN superiority emerges when the disrupted satellite channel is the last leg of a congested path, thanks to the DTN custody transfer mechanism and store and forward capability
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
